What is NPSHa in pump hydraulics?

• A. The suction head available to supply vapor-prone fluid (including atmospheric, static, and loss terms minus vapor pressure).
• B. The minimum suction head required by the pump to avoid cavitation.
• C. The discharge head of the pump.
• D. The vapor pressure of the liquid.

Answer: (A)

NPSHa is the net positive suction head available at the pump suction, which represents the pressure energy in the fluid on the suction side that can be used by the pump without cavitation. It combines the supply pressure (often atmospheric pressure), the static suction head (the vertical height the fluid must be lifted), and the losses in the suction line, all converted to head, and then subtracts the vapor pressure head of the liquid. This gives a margin above the liquid’s vapor pressure that helps prevent the formation of vapor pockets inside the pump.

This is the best description because it directly defines the amount of suction energy that is actually available to the pump to keep the liquid from vaporizing. Cavitation occurs when the pressure in the pump eye drops to the vapor pressure, so maintaining NPSHa above the required level is essential.

The other options don’t fit because: the minimum suction head required by the pump refers to NPSHr, not what is available; the discharge head is the energy added on the output side, not the suction side; and the vapor pressure is just one part of the calculation, not the overall available suction head.